This invention relates to methods for the controlled generation of ammonia or other gases from an ammoniated metal salt or the like at substantially ambient temperature by applying microwave energy to the salt. The methods are useful in, inter alia, adsorption type refrigeration systems where ammonia is used as a refrigerant and the salt is used as a sorbent bed.
Adsorption type refrigeration systems are described generally in Perry's Chemical Engineers' Handbook, Sixth Edition (R. H. Perry, D. W. Green, and J. O. Maloney, eds., McGraw-Hill, 1984). The chemical components of such a system are comprised of (1) a sorbate which functions as a refrigerant and is normally a gas at room temperature and atmospheric pressure, (2) a solid or liquid sorbent onto which the sorbate adsorbs, and (3) the sorbate:sorbent complex. The cooling effect in these systems is achieved by operation of an adsorption/desorption cycle whereby the sorbate is alternatingly adsorbed onto and desorbed from the sorbent. The desorption step is performed on the sorbate:sorbent complex causing the sorbate to be separated from the complex as a gas. Desorption also regenerates the sorbent thereby allowing the cycle to be repeated. The separated gas may be transferred to a condenser and, once condensed, then transferred to an evaporator where vaporization will occur with the desired, concomitant cooling of the surrounding atmosphere.
In a conventional refrigeration system, desorption is achieved by heating the sorbate:sorbent complex. The amount of heat required will depend on properties of the complex such as its specific heat and the energy of association between sorbate and sorbent. The greater the heat required to effect desorption, the lower will be the efficiency R of the refrigeration system, which is a ratio of the useful refrigerating effect divided by the heat input required during the refrigeration cycle: ##EQU1##
It is thus a disadvantage for refrigeration systems to require high heat input for desorption. Another disadvantage of heating the sorbate:sorbent complex desorption is that the regenerated sorbent must first be cooled before the sorption cycle is repeated. Substantial cooling of the sorbent is needed because adsorption occurs much more effectively at temperatures well below those required for desorption. Since both the heating of the complex and cooling of the resulting regenerated sorbent expend significant energy and time, it would be desirable to develop new systems in which the desorption step can be effected in a shorter time and with less energy.
U.S. Pat. No. 4,848,994 describes a refrigeration system which uses ammoniated complex compounds. In this system, the refrigerant sorbate is ammonia and the sorbent is a metal salt which is capable of forming an ammoniated complex. Included among the disclosed salts are metal chlorides or bromides in which the metal is an alkaline earth metal, chromium, manganese, iron, cobalt, nickel, cadmium, tantalum or rhenium. In the ammoniated complex, the number of ammonia molecules that are complexed with the salt may vary from two to eight per molecule of salt depending on the salt. Desorption of the ammonia is reported to be effected by heating the complex.
Heat induced desorption of ammonia from ammoniated salts has also been described by R. M. Buffington in Refrigeration Engineering 26, 137 (1933).
U.S. Pat. No. 4,312,640 describes the desorption of polar gases from a desiccant or sorbent using microwave energy. Unlike the adsorbed polar gas, the sorbent material is transparent to microwave energy so that desorption may occur without appreciable heating of the sorbent. Examples of sorbents that are described as microwave transparent include silica gel, Mobil Sorbeads, magnesium sulfate, calcium sulfate and natural and synthetic zeolites. It is stated that useful sorbents are those having strongly bound water of hydration, such as molecular sieves or alumina. The patent states that the water of hydration is not removed from the sorbent in the desorption process it describes.
U.S. Pat. No. 5,227,598 describes a system for regenerating adsorbent material. The patent discloses the use of microwave heating of adsorbents to a temperature sufficient to desorb the sorbates from the adsorbent.